Different retroviruses have been isolated from various mammalian species by cocultivation of tissues with heterologous cells. These isolates include five different primate viruses: type C viruses from two Old World monkeys (baboon and macaque) and from a New World primate (owl monkey); and type D viruses from Old World (langur) and New World (squirrel monkey) primates. These isolates are all endogenous in their respective species, and are present in multiple copies in the cellular DNA of these and related species. The squirrel monkey, a New World primate, contains multiple copies of endogenous type D retroviral gene sequences in the cellular DNA of all its tissues. Partially homologous viral gene sequences are also found in the cellular DNA of a New World carnivore, the skunk. We therefore conclude that this class of viruses has, under natural conditions, been transmitted between the germ lines of these evolutionarily distant species. Type C viruses thus seem to have a propensity for trans-species infection and subsequent integration into the germ cells of evolutionarily distant species.